We are critically testing the hypothesis of selective neutrality of naturally occuring electrophoretic enzyme varients using Escherichia coli in chemostats. We are currently studying fine different alleles of gnd (6-phosphogluconate dehydrogenase) for selective differences when the strains are competing for gluconic acid in chemostats. Two different backgrounds are used: an edd- background which forces all energy metabolism to be channeled via 6-phosphogluconate dehydrogenase and a edd plus background, the natural background, where only 25% of the energy metabolism is channeled through the 6-phosphogluconate dehydrogenase. We are also studying the simple enzymatic properties of these alleles as Km, Vmax, KI etc. to determine if functional differences can be found and then correlated with any selective differences found. So far, biochemical differences have been found and seem to determine the selective differences seen in edd- backgrounds. No selective differences can be found in the edd plus backgrounds.